A method for the targeted setting of an internal pressure in a cavity of a MEMS element is described in PCT Appl. No. WO 2015/120939 A1. A narrow access channel to a MEMS cavity is created in a cap wafer or in a sensor wafer. The cavity is flooded with the desired gas at the desired internal pressure via the access channel. The area around the access channel is subsequently locally heated using a laser. The substrate material hereby liquefies locally, and upon solidification hermetically seals the access channel.
Such a method is primarily used when it is desired to create two cavities in a MEMS element, and a different internal pressure is to be provided in the two cavities (FIG. 1).
This is necessary when it is desired to combine, for example, an acceleration sensor with a rotation rate sensor. The acceleration sensor requires a high internal pressure for optimal functioning, whereas a rotation rate sensor requires a rather small internal pressure. This method allows the particular optimal internal pressure to be set in each cavity.
When a very low internal pressure in a cavity is unachievable as the result of outgassing during process control, the method may also be utilized to subsequently set the internal pressure.
In one specific configuration in which only one MEMS element is implemented and the evaluation circuit is provided separately, the creation of access channel (1) may occur at the same time that electrical contact surfaces (2) are exposed. A relatively thin cap wafer is generally used in this type of configuration. After cap wafer (3) is bonded to sensor wafer (4), by use of a photomask and a trenching process it is possible in one step to create a narrow access opening to a first cavity as well as a large access opening to the contact surfaces.
German Patent No. DE 102011103516 B4 describes encapsulating a MEMS structure with a deposited polycrystalline silicon layer, and then creating an access channel into the polycrystalline silicon layer, using a laser drilling process. A defined atmosphere is then set in the MEMS cavity, and the access channel is sealed using a laser sealing process. Creating the access channel is cost-effective compared to a pure trenching process via which only one access channel is created. It is disadvantageous that in a laser drilling process, smoke always arises that may damage the MEMS structures. It is also disadvantageous that the laser drilling process is not very selective for different materials. This means that, regardless of the material, during laser drilling not only is an access hole created through the polycrystalline silicon layer, but at the same time a hole is also drilled into the underlying layer.